Used as a DC voltage power supply, a battery pack may include a group of battery cells connected in series. Charging and discharging the battery pack through normal operation over time may result in cell-to-cell variations in cell voltages. When one or more cells in a series string charge faster or slower than the others, an unbalanced condition may occur.
FIG. 1 illustrates a conventional cell balancing circuit 100. Multiple cells connected in series include a first cell 102 and a second cell 103. The positive terminal (anode) of the cell 102 is coupled to a controller 110 at a terminal BAT1 through a resistor 108. The negative terminal (cathode) of the cell 102 is coupled to the controller 110 at a terminal BAT0 through a resistor 106. Inside the controller 110, an internal shunt path (bypass path) is parallel-connected with the cell 102. The internal shunt path includes a bleeding control switch 104. The bleeding control switch 104 is controlled by the controller 110 through a control signal DRV1. Similarly, the anode of the cell 103 is coupled to the controller 110 at the terminal BAT2 through a resistor 112. The cathode of the cell 103 is coupled to the controller 110 at the terminal BAT1 through the resistor 108. Inside the controller 110, an internal shunt path is parallel-connected with the cell 103. The internal shunt path includes a bleeding control switch 116. The bleeding control switch 116 is controlled by the controller 110 through a control signal DRV2.
When an unbalanced condition occurs, for example, when a voltage of the cell 102 is greater than that of any other cell in the battery pack, the controller 110 may turn on the switch 104 to enable a bypass current (shunt current) to flow through the internal shunt path, and thus charging of the cell 102 can be slowed down and the cell voltages can be balanced in the battery pack. One of the disadvantages of this method is that the heat generated by the bypass current may accumulate inside the controller 110 and may cause damage to the controller 110.
FIG. 2 shows another conventional cell balancing circuit 200. Elements labeled the same as in FIG. 1 have similar functions. The anode of the cell 102 is coupled to a controller 210 at a terminal BAT1 through a resistor 208. The cathode of the cell 102 is coupled to the controller 210 at a terminal BAT0 through a resistor 206. An external shunt path is parallel-connected with the cell 102. The shunt path includes a resistor 201 and a bleeding control switch 204, e.g., an N channel metal oxide semiconductor field effect transistor (NMOSFET), connected in series with the resistor 201. The bleeding control switch 204 is controlled by the controller 210 via a dedicated pin CB1. Similarly, the anode of the cell 103 is coupled to the controller 210 at the terminal BAT2 through a resistor 212. The cathode of the cell 103 is coupled to the controller 210 at the terminal BAT1 through the resistor 208. An external shunt path is parallel-connected with the cell 103. The external shunt path includes a resistor 214 and a bleeding control switch 216, e.g., an NMOSFET, connected in series with the resistor 214. The bleeding control switch 216 is controlled by the controller 210 via a dedicated pin CB2.
When an unbalanced condition occurs, for example, when a voltage of the cell 102 is greater than that of any other cell in the battery pack, the controller 210 may turn on the switch 204 to enable a bypass current to flow through the external shunt path, and thus charging of the cell 102 can be slowed down and the cell voltages can be balanced in the battery pack. One of the disadvantages of this method is that each cell needs an extra pin (e.g., CB1 for cell 102, CB2 for cell 103) to control a corresponding bleeding control switch, which increases the cost.
FIG. 3 shows another conventional cell balancing circuit 300. Elements labeled the same as in FIG. 1 and FIG. 2 have similar functions. In the conventional cell balancing circuit 300, the anode of the cell 102 is coupled to a controller 310 at a terminal BAT1 through a resistor 308. The cathode of the cell 102 is coupled to the controller 310 at a terminal BAT0 through a resistor 306. An external shunt path is parallel-connected with the cell 102. The shunt path can include a resistor 301 and a bleeding control switch (e.g., a bipolar junction transistor 302) connected in series with the resistor 301. The conductance status of the bipolar junction transistor 302 is determined by the voltage drop across the resistor 306. Similarly, the anode of the cell 103 is coupled to the controller 310 at a terminal BAT2 through a resistor 313. The cathode of the cell 103 is coupled to the controller 310 at the terminal BAT1 through the resistor 308. An external shunt path is parallel-connected with the cell 103. The shunt path can include a resistor 314 and a bleeding control switch (e.g., a bipolar junction transistor 304) connected in series with the resistor 314. The conductance status of the bipolar junction transistor 304 is determined by the voltage drop across the resistor 308.
In the controller 310, an internal switch 312 is coupled between the terminal BAT1 and the terminal BAT0. An internal switch 316 is coupled between the terminal BAT2 and the terminal BAT1. The internal switch 312 is controlled by the controller 310 through a control signal DRV1. The internal switch 316 is controlled by the controller 310 through a control signal DRV2.
In FIG. 3, in order to conduct the shunt path of the cell 102, the internal switch 312 is turned on to enable a current I2 flowing from the anode of the cell 102 through the resistor 308 and the terminal BAT1 into the controller 310. In order to conduct the shunt path of the cell 103, the internal switch 316 is turned on to enable a current I3 flowing out of the controller 310 through the terminal BAT1, the resistor 308 to the cathode of the cell 103. As a result, there may be a confliction of the current direction regarding the current flowing through the resistor 308. Since I2 and I3 flow in opposite directions, if the level of I2 and the level of I3 are the same, the voltage drop across the resistor 308 will be zero such that the bipolar junction transistor 304 can not be turned on. Therefore, this conventional cell balancing circuit 300 is not able to balance two neighboring cells at the same time.
Moreover, the bypass current flowing through a shunt path is limited by a base current of a corresponding bipolar junction transistor. For example, if the cell 103 is unbalanced, the internal switch 316 is turned on and the internal switch 312 is turned off. A current I1 flows from the anode of the cell 103 through the resistor 313, the terminal BAT2 and the internal switch 316 to the terminal BAT1. The base current IB of the bipolar junction transistor 304 flows into the bipolar junction transistor 304. The current I3 flows through the resistor 308 to the cathode of the cell 103. The bypass current IBLD flowing through the shunt path of the cell 103 can be given by:IBLD=β·IB,  (1)where β is the common-emitter current gain of the bipolar junction transistor 304. In order to acquire a relatively large bypass current IBLD, the base current IB needs to be increased. However, since the current I1 is the summation of the base current IB and the current I3 flowing through the resistor 308, a relatively large base current IB may result in a relatively small current I3. On the other hand, the current I3 needs to be large enough such that the voltage drop across the resistor 308 is large enough to turn on the bipolar junction transistor 304. Therefore, the bypass current IBLD flowing through the shunt path of the cell 103 is limited by the base current IB of the bipolar junction transistor 304.
Furthermore, the conventional cell balancing circuit 300 may not be suitable for balancing cells having relatively low cell voltages. Assume that the resistance of the resistor 308 and the resistance of the resistor 306 are the same. If the cell 102 is unbalanced, the controller 310 turns on the internal switch 312 by the control signal DRV1. Neglecting the voltage drop across the internal switch 312, the voltage drop across the resistor 306 is only half of the voltage of the cell CELL-1. If the voltage of the cell CELL-1 is too low, the bleeding control switch (the bipolar junction transistor 302) in the shunt path may not be turned on.